1. Field of the Invention
The invention refers to a wheelchair with middle wheel drive, the wheelchair comprising two middle wheels separately drivable by motoric power, at least one front wheel and at least one rear wheel, the rear wheel or the front wheel being located at an elevated position above the plane of motion. The term wheelchair employed in this description is understood in the general sense of motor driven vehicles for handicapped persons.
2. The Related Art
U.S. Pat. No. 5,904,214, for example, discloses a wheelchair with middle wheel drive comprising two middle wheels, two front wheels in the form of castors, and a rear wheel which can be driven by a motor and is located at a position above the plane of motion. Each of the middle wheels is separately driven by a motor. This has the advantage that the wheelchair can be turned in narrow spaces, e.g. in an elevator cabin. To turn on the spot the operator actuates the controls to have the middle wheels turning in opposite directions to each other. The front wheels should not impede turning. Therefore, the front wheels are in form of swiveling wheels or castors. This wheelchair has the disadvantage that obstacles, if they exceed a certain height cannot be surmounted at slow speed. Further, the described wheelchair has little road grip on a snow covered ground and therefore is not anymore in a position to move up a ramp which has a relatively high gradient.
U.S. Pat. No. 6,712,369 describes a wheelchair with two middle wheels, two swiveling front wheels and two rear wheels. To drive the middle wheels a single motor is provided, but it is mentioned in the specification that further suitable driving means may be employed. The rear wheels have no driving means, are not swivelable and are not in contact with the plane floor. However, they can be moved upward against the force of springs. In contrast to the previously described wheelchair the rear wheels have no motoric drive. The wheelchair according the U.S. Pat. No. 6,712,369 is not capable to surmount at slow speed relatively high obstacles. With some skill such obstacles can be surmounted. If the wheelchair is strongly accelerated the springs of the rear wheels will be pressed together and the front wheels will be lifted so that they can surmount also a relatively high obstacle. However, most users of wheelchairs have an uncomfortable feeling in making such maneuvers. Some users of wheelchairs may even not dare such maneuvers, because they could be injured by the occurring shakes and jolts. It is further of disadvantage that on bad Toad conditions, e.g. because of snow, maneuverability of the wheelchair leaves much to be desired, because it has only two driven wheels.
In WO 96/15752 it was suggested on page 15, line 10 with reference to FIG. 14 to locate the front wheels at an elevated position, that is without contact to the floor, as also suggested later on in U.S. Pat. No. 6,129,165. This facilitates surmounting of an obstacle. This is not possible on driving backward, because only the front wheels are at an elevated position.
It is therefore an advantage of the present invention to create a wheelchair with middle wheel drive being capable, both on driving forward and backward, of surmounting relatively high obstacles even at relatively low speed. Also e.g. on snow covered pavement it should have a good driving behavior. Nevertheless, it should have the advantages of the middle wheel drive, in particular the capability to turn around in narrow spaces.